Noncovalent Functionalization of Single-Walled Carbon Nanotubes with a Photocleavable Polythiophene Derivative

Single-walled carbon nanotubes (SWCNTs) have received extensive research attention owing to their extraordinary optical, electrical, and mechanical properties, which make them particularly attractive for application in optoelectronic devices. However, SWCNTs are insoluble in almost all solvents. Therefore, developing methods to solubilize SWCNTs is crucial for their use in solution-based processes. In this study, we developed a photocleavable polythiophene-derivative polymer dispersant for SWCNTs. The noncovalent surface functionalization of SWCNTs with a polymer allows their dispersal in tetrahydrofuran. The resultant solution-processed polymer/SWCNT composite film undergoes a hydrophobic-to-hydrophilic change in surface properties upon light irradiation (313 nm) because hydrophilic carboxyl groups are formed upon photocleavage of the hydrophobic solubilizing units in the polymer. Furthermore, the photocleaved composite film displays a 38-fold increase in electrical conductivity. This is due to the removal of the solubilizing unit, which is electrically insulating.

While there are a variety of applications for SWCNTs, they also present certain drawbacks. For instance, SWCNT surfaces are hydrophobic, leading to unfavorable aggregation. Accordingly, SWCNTs are poorly dispersible and quickly precipitate in most organic solvents. Therefore, the development of new SWCNT-dispersion strategies is required for their use in solution-based processes and applications.
Noncovalent functionalization provides a means for the solution processing of SWC-NTs [9,10]. When a dispersant, such as a surfactant, polymer, or biomaterial, is attached to an SWCNT surface through hydrophobic and/or π-π interactions, the uniform dispersion of the dispersant/SWCNT composite can occur without deforming its sp 2 carbon structure [11,12]. Conjugated polymers are good candidates for use as functional dispersants [13] because extended π-structures bearing solubilizing units (e.g., alkyl chains) can bind noncovalently through π-π interactions to SWCNTs, allowing the formation of SWCNT suspensions. Subsequently, such suspensions can be used to make solution-processed SWCNT films using coating and printing techniques [14]. However, the solubilizing conjugated polymer remains attached to the surface of the SWCNTs, meaning that the resulting film can be damaged upon exposure to the solvent in which it was made. Therefore, the fabrication of insoluble SWCNT films using functional dispersants would benefit their practical application [15].
In this study, we developed the polythiophene dispersant PC 56 T 44 , which features a photocleavable solubilizing unit (Figure 1a). Photoresponsive (coumarin-4-yl)methyl groups have been reported as cage compounds for biomedical applications [16,17]. We introduced an octyloxy chain as a solubilizing unit in the coumarin group. Exposure to light leads to the removal of the coumarin group, including the solubilizing unit. The novelty of this approach is that the photocleaved unit allows not only the formation of an insoluble PC 56 T 44 /SWCNT composite film, but it also changes the surface properties of the film. Upon photocleavage, the surface of the composite film changes from hydrophobic to hydrophilic. This change is due to the formation of hydrophilic carboxyl (COOH) groups on the polymer attached to the surface of the SWCNTs. Furthermore, the electrical conductivity of the photocleaved PC 56 T 44 /SWCNT composite film is 38 times that of the pristine film because the solubilizing unit is an electrical insulator. can bind noncovalently through π-π interactions to SWCNTs, allowing the formation of SWCNT suspensions. Subsequently, such suspensions can be used to make solution-processed SWCNT films using coating and printing techniques [14]. However, the solubilizing conjugated polymer remains attached to the surface of the SWCNTs, meaning that the resulting film can be damaged upon exposure to the solvent in which it was made. Therefore, the fabrication of insoluble SWCNT films using functional dispersants would benefit their practical application [15].
In this study, we developed the polythiophene dispersant PC56T44, which features a photocleavable solubilizing unit (Figure 1a). Photoresponsive (coumarin-4-yl)methyl groups have been reported as cage compounds for biomedical applications [16,17]. We introduced an octyloxy chain as a solubilizing unit in the coumarin group. Exposure to light leads to the removal of the coumarin group, including the solubilizing unit. The novelty of this approach is that the photocleaved unit allows not only the formation of an insoluble PC56T44/SWCNT composite film, but it also changes the surface properties of the film. Upon photocleavage, the surface of the composite film changes from hydrophobic to hydrophilic. This change is due to the formation of hydrophilic carboxyl (COOH) groups on the polymer attached to the surface of the SWCNTs. Furthermore, the electrical conductivity of the photocleaved PC56T44/SWCNT composite film is 38 times that of the pristine film because the solubilizing unit is an electrical insulator.

Polymer Dispersants for SWCNT
Syntheses of the polymeric dispersants PC56T44 and PT have been described elsewhere [18]. PC56T44 was afforded with a number-average molecular weight (Mn) of 17,000 and a polydispersity (weight-average molecular weight (Mw)/Mn) of 1.4. The corresponding values for PT were Mn: 6800 and Mw/Mn: 1.7.
The photocleavage behavior of PC56T44 was investigated by NMR and Fourier-transform infrared (FT-IR) spectroscopy. A PC56T44 film was prepared by drop-casting a CHCl3

Polymer Dispersants for SWCNT
Syntheses of the polymeric dispersants PC 56 T 44 and PT have been described elsewhere [18]. PC 56 T 44 was afforded with a number-average molecular weight (M n ) of 17,000 and a polydispersity (weight-average molecular weight (M w )/M n ) of 1.4. The corresponding values for PT were M n : 6800 and M w /M n : 1.7.
The photocleavage behavior of PC 56 T 44 was investigated by NMR and Fourier-transform infrared (FT-IR) spectroscopy. A PC 56 T 44 film was prepared by drop-casting a CHCl 3 solu-tion (15 mg mL −1 ) on a fused silica substrate. The polymer film was irradiated at 313 nm (light intensity: 5 mW cm −2 ) for 5 h using a high-pressure mercury lamp (REX-250, Asahi Spectra Co., Ltd., Tokyo, Japan) equipped with a glass filter (HQBP313, Asahi Spectra). The unreacted polymer and cleaved coumarin groups were removed by immersion in CHCl 3 /hexane (1:3 (v/v)). After drying, the resulting polymer on the substrate was dissolved in CDCl 3 , yielding the photoirradiated NMR sample.
An FT-IR sample was prepared by drop-casting a CHCl 3 solution (15 mg mL −1 ) onto a KBr plate. After irradiation at 313 nm for 5 h, the photoirradiated FT-IR sample was obtained by immersion in CHCl 3 /hexane (1:3 (v/v)) to remove the unreacted polymer and cleaved coumarin groups.

Dispersal of SWCNTs in THF
PC 56 T 44 (1 mg) was dissolved in THF (10 mL) and SWCNTs (1 mg) were added to the resulting solution. The polymer/SWCNT mixture was ultrasonicated for 1 h using a tip-type ultrasonic homogenizer (Branson Sonifier 250, Branson Ultrasonics Co., Brookfield, CT, USA; power output: 20 W) in an ice bath. The suspension was centrifuged at 4000 rpm for 30 min using a Microfuge 16a centrifuge (Beckman Coulter Inc., Brea, CA, USA). A suspension of PC 56 T 44 /SWCNT (1:1 (w/w)) composite in THF was obtained upon collection of the supernatant layer.
A suspension of PT/SWCNT (1:1 (w/w)) composite was prepared in a similar way as that described for the PC 56 T 44 /SWCNT composite.

Preparation of PC 56 T 44 /SWCNT Composite Films
The substrates were cleaned with acetone and IPA by ultrasonication twice before preparing the samples. The substrates were treated with an ultraviolet-ozone cleaner (ASM1101N, Asumi Giken Ltd., Tokyo, Japan) for 10 min. A PC 56 T 44 /SWCNT (1:1 (w/w)) composite film was fabricated by drop-casting a THF solution (0.2 mg mL −1 ) onto the substrate. The obtained film was dried at 25 • C for 15 h. The PC 56 T 44 /SWCNT composite film was irradiated at 313 nm (light intensity: 5 mW cm −2 ) for 5 h. Photoirradiation was performed at 25 • C under nitrogen using a temperature controller (FP90 central processor equipped with a FP-82HT hot stage, Mettler Toledo, Columbus, OH, USA). After irradiation, the unreacted polymer and cleaved coumarin groups were removed by immersion in CHCl 3 /hexane (1:3 (v/v)).
Water contact angle measurements were obtained using a DropMaster 500 contact angle meter (Kyowa Interface Science Co., Ltd., Tokyo, Japan) equipped with a chargecoupled device camera. The water contact angle was measured immediately after 1 µL of water was dropped onto the PC 56 T 44 /SWCNT composite film with a syringe.
Film thickness was measured with a Dektak surface profiler (Bruker Co., Billerica, MA, USA).
Conductivity measurements were performed using a Keithley 2400 source meter (Tektronix Inc., Beaverton, OR, USA).  Figure 1a shows the chemical structures of the polythiophene derivatives PC 56 T 44 and PT. PC 56 T 44 includes a (coumarin-4-yl)methyl moiety as a photocleavable group and an octyloxy side chain as a solubilizing unit. PT possesses a methyl acetate group without a photoreactive group. The absorption spectra of PC 56 T 44 and PT in THF are shown in Figure 1b. The maximum absorption wavelength (λ max ) values for PC 56 T 44 are 322 and 381 nm. The former λ max is due to the S 1 → S 0 transition of the coumarin group [19], while the latter is due to the π-π* transition of a polythiophene unit in a random coil structure [20]. Since PT also has the π-π* transition at 383 nm, PC 56 T 44 and PT show similar π-conjugated structures in the polythiophene chain.

Photocleavage of PC 56 T 44
A change in the absorption spectrum of PC 56 T 44 in THF occurs upon photoirradiation at 313 nm ( Figure 2a). As the irradiation time increases, λ max at 322 nm decreases. After irradiation for 5 h, a photostationary state is reached. These results indicate that photoirradiation leads to photocleavage of the coumarin group bearing the solubilizing unit. As the irradiation time increases, the absorbance at 381 nm decreases. This may be due to the photocleavage of the bulky coumarin group, which changes the polythiophene conformation. Thus, the main chain shows a decrease in the conjugation length.
Nanomaterials 2022, 12, x FOR PEER REVIEW 4 of 9 Figure 1a shows the chemical structures of the polythiophene derivatives PC56T44 and PT. PC56T44 includes a (coumarin-4-yl)methyl moiety as a photocleavable group and an octyloxy side chain as a solubilizing unit. PT possesses a methyl acetate group without a photoreactive group. The absorption spectra of PC56T44 and PT in THF are shown in Figure  1b. The maximum absorption wavelength (λmax) values for PC56T44 are 322 and 381 nm. The former λmax is due to the S1  S0 transition of the coumarin group [19], while the latter is due to the π-π* transition of a polythiophene unit in a random coil structure [20]. Since PT also has the π-π* transition at 383 nm, PC56T44 and PT show similar π-conjugated structures in the polythiophene chain.

Photocleavage of PC56T44
A change in the absorption spectrum of PC56T44 in THF occurs upon photoirradiation at 313 nm ( Figure 2a). As the irradiation time increases, λmax at 322 nm decreases. After irradiation for 5 h, a photostationary state is reached. These results indicate that photoirradiation leads to photocleavage of the coumarin group bearing the solubilizing unit. As the irradiation time increases, the absorbance at 381 nm decreases. This may be due to the photocleavage of the bulky coumarin group, which changes the polythiophene conformation. Thus, the main chain shows a decrease in the conjugation length. The FT-IR spectra of PC56T44 were obtained before and after irradiation to investigate its photocleavage behavior (Figure 2b). PC56T44 shows ester group absorbances at 1261 (C-O) and 1734 cm −1 (C=O) before irradiation. After irradiation, a broadened peak appeared at around 3200-3400 cm −1 , which is attributed to a stretching mode of a carboxyl (COOH) group (Figure 2(b2)). 1 H NMR analysis revealed that irradiation at 313 nm leads to photocleavage of the coumarin units ( Figure S1 and S2). The integration value for the methylene groups in the coumarin unit (5.20 ppm) decreased from 2.46 to 1.77. Because the integration value for the methyl acetate group (3.68 ppm) did not change, 28% of the thiophene units with the coumarin groups were changed in the polymer chain. Thus, there was a 16% yield of COOH groups in the polymer chain upon photocleavage (Figure 2c). We also found that a broadening of the peak around 1730 cm -1 occurred in the FT-IR spectrum after irradiation (Figure 2(b2)). This may be attributed to the overlapped peaks of ester and COOH The FT-IR spectra of PC 56 T 44 were obtained before and after irradiation to investigate its photocleavage behavior (Figure 2b). PC 56 T 44 shows ester group absorbances at 1261 (C-O) and 1734 cm −1 (C=O) before irradiation. After irradiation, a broadened peak appeared at around 3200-3400 cm −1 , which is attributed to a stretching mode of a carboxyl (COOH) group (Figure 2(b2)). 1 H NMR analysis revealed that irradiation at 313 nm leads to photocleavage of the coumarin units (Figures S1 and S2). The integration value for the methylene groups in the coumarin unit (5.20 ppm) decreased from 2.46 to 1.77. Because the integration value for the methyl acetate group (3.68 ppm) did not change, 28% of the thiophene units with the coumarin groups were changed in the polymer chain. Thus, there was a 16% yield of COOH groups in the polymer chain upon photocleavage (Figure 2c). We also found that a broadening of the peak around 1730 cm −1 occurred in the FT-IR spectrum after irradiation Nanomaterials 2022, 12, 52 5 of 9 ( Figure 2(b2)). This may be attributed to the overlapped peaks of ester and COOH groups. Because the photocleavage rate was insufficient (16%), the peak of the COOH group around 1710 cm −1 could not be observed clearly.

Dispersion of SWCNT Using Noncovalent Functionalization
Ultrasonication of polythiophene derivatives and SWCNT in THF yielded suspensions (Figure 3a). The color of the PC 56 T 44 /SWCNT composite suspension is darker than that of the PT/SWCNT composite. This indicates that PC 56 T 44 has a better ability to disperse SWCNTs than PT. groups. Because the photocleavage rate was insufficient (16%), the peak of the COOH group around 1710 cm -1 could not be observed clearly.

Dispersion of SWCNT Using Noncovalent Functionalization
Ultrasonication of polythiophene derivatives and SWCNT in THF yielded suspensions (Figure 3a). The color of the PC56T44/SWCNT composite suspension is darker than that of the PT/SWCNT composite. This indicates that PC56T44 has a better ability to disperse SWCNTs than PT.  Figure 3b shows the absorption spectra of the suspensions. The higher absorbance of the PC56T44/SWCNT composite is evidence of more dispersed SWCNTs. Thus, the π-π interactions between the aromatic coumarin in PC56T44 and the SWCNTs result in improved dispersion. The absorption spectra of the suspensions in the visible and near-infrared regions are consistent with the electronic structure of SWCNTs, corresponding to optical transitions for the first (S11, 940−1300 nm) and second (S22, 620−940 nm) semiconducting SWCNTs [21].
The Raman spectrum of PC56T44/SWCNT (1:1 (w/w)) presented two characteristic bands: the disorder (D)-band at 1317 cm -1 and the graphitic (G)-band at 1586 cm -1 ( Figure  4a). The D-band is related to the presence of defects, corresponding to sp 3 -hybridized carbon in the hexagonal frameworks of the nanotubes. The G-band is attributed to the vibration of sp 2 carbons in graphitic layers [23]. The D-and G-band intensity ratio (ID/IG) indicates the level of defects in SWCNTs. The ID/IG value for PC56T44/SWCNT was 0.12, indicating that the SWCNTs had almost no structural defects after dispersion.  Figure 3b shows the absorption spectra of the suspensions. The higher absorbance of the PC 56 T 44 /SWCNT composite is evidence of more dispersed SWCNTs. Thus, the π-π interactions between the aromatic coumarin in PC 56 T 44 and the SWCNTs result in improved dispersion. The absorption spectra of the suspensions in the visible and near-infrared regions are consistent with the electronic structure of SWCNTs, corresponding to optical transitions for the first (S 11 , 940−1300 nm) and second (S 22 , 620−940 nm) semiconducting SWCNTs [21].
The broadening of an absorption peak is proportional to the degree of aggregation in a sample. Accordingly, to evaluate dispersion behavior, we measured the full width at half maximum (FWHM) at 1153 nm in the S 11 region. The FWHM for the PC 56 T 44 /SWCNT composite was 39.4 meV. The FWHM for the poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6'-{2,2'-bipyridine})]/(6,5) SWCNT composite in the S 11 region was 22.4 meV [22], indicating that our composite tended to show SWCNT aggregation in the suspension.
The Raman spectrum of PC 56 T 44 /SWCNT (1:1 (w/w)) presented two characteristic bands: the disorder (D)-band at 1317 cm −1 and the graphitic (G)-band at 1586 cm −1 (Figure 4a). The D-band is related to the presence of defects, corresponding to sp 3 -hybridized carbon in the hexagonal frameworks of the nanotubes. The G-band is attributed to the vibration of sp 2 carbons in graphitic layers [23]. The D-and G-band intensity ratio (I D /I G ) indicates the level of defects in SWCNTs. The I D /I G value for PC 56 T 44 /SWCNT was 0.12, indicating that the SWCNTs had almost no structural defects after dispersion. The Raman spectrum of PC56T44/SWCNT (1:1 (w/w)) presented two characteristic bands: the disorder (D)-band at 1317 cm -1 and the graphitic (G)-band at 1586 cm -1 ( Figure  4a). The D-band is related to the presence of defects, corresponding to sp 3 -hybridized carbon in the hexagonal frameworks of the nanotubes. The G-band is attributed to the vibration of sp 2 carbons in graphitic layers [23]. The D-and G-band intensity ratio (ID/IG) indicates the level of defects in SWCNTs. The ID/IG value for PC56T44/SWCNT was 0.12, indicating that the SWCNTs had almost no structural defects after dispersion.  (Figure 4b). Since the SWCNT diameters were approximately 0.7−1.4 nm, the bundles of SWCNT originated from entangled structures. Since the surfaces of SWCNTs were smooth and uniform, the PC 56 T 44 attached to the surface of SWCNTs brought about π−π interactions between the polythiophene units and the SWCNTs.

Morphological Change at the Surface of the Solution-Processed SWCNT Film
We prepared a drop-cast film of the PC 56 T 44 /SWCNT composite on a fused silica substrate (Figure 5a). The film thickness was~1.2 µm. Photocleavage of PC 56 T 44 in the SWCNT film was investigated by SEM (Figure 5b). The as-prepared PC 56 T 44 /SWCNT composite film showed network structures. The composite bundles were 30−40 nm in diameter. However, the SEM image is not clear owing to the aggregation of SWCNTs covered with the polymer (Figure 5(b1)). In contrast, network structures were observed clearly after photoirradiation at 313 nm for 5 h. The diameters of the composite bundles decreased by 40% to 60% (Figure 5(b2)). These results suggest that the decrease in diameter was due to photocleavage of the solubilizing unit in PC 56 T 44 on the surface of the SWCNTs. We also found that the PC 56 T 44 /SWCNT composite film was insoluble in CHCl 3 , dichloromethane, and THF after photoirradiation, owing to the photocleavage of the solubilizing unit.  (Figure 4b). Since the SWCNT diameters were approximately 0.7−1.4 nm, the bundles of SWCNT originated from entangled structures. Since the surfaces of SWCNTs were smooth and uniform, the PC56T44 attached to the surface of SWCNTs brought about π−π interactions between the polythiophene units and the SWCNTs.

Morphological Change at the Surface of the Solution-Processed SWCNT Film
We prepared a drop-cast film of the PC56T44/SWCNT composite on a fused silica substrate (Figure 5a). The film thickness was ~1.2 μm. Photocleavage of PC56T44 in the SWCNT film was investigated by SEM (Figure 5b). The as-prepared PC56T44/SWCNT composite film showed network structures. The composite bundles were 30−40 nm in diameter. However, the SEM image is not clear owing to the aggregation of SWCNTs covered with the polymer (Figure 5(b1)). In contrast, network structures were observed clearly after photoirradiation at 313 nm for 5 h. The diameters of the composite bundles decreased by 40% to 60% (Figure 5(b2)). These results suggest that the decrease in diameter was due to photocleavage of the solubilizing unit in PC56T44 on the surface of the SWCNTs. We also found that the PC56T44/SWCNT composite film was insoluble in CHCl3, dichloromethane, and THF after photoirradiation, owing to the photocleavage of the solubilizing unit. Photocleavage of the solubilizing unit in PC56T44 afforded COOH groups. These groups made the SWCNT surface hydrophilic. When a surface changes from hydrophobic to hydrophilic, the water contact angle decreases. Photocleavage of the solubilizing unit decreased the water contact angle (Figure 6a). The water contact angle of the PC56T44/SWCNT composite film before irradiation was 93°. This result indicates that the surfaces of the SWCNTs were covered with PC56T44 through noncovalent functionalization. In contrast, the water contact angle after irradiation at 313 nm was 43°. Since the water contact angle for the pristine SWCNTs was around 85° [24], the change in the surface composition of the SWCNT film led to a change in the water contact angle. We also found that the formation of COOH groups upon photocleavage increased surface hydrophilicity (Figure 6b). Therefore, the composite film became hydrophilic after photocleavage of the hydrophobic solubilizing unit. Photocleavage of the solubilizing unit in PC 56 T 44 afforded COOH groups. These groups made the SWCNT surface hydrophilic. When a surface changes from hydrophobic to hydrophilic, the water contact angle decreases. Photocleavage of the solubilizing unit decreased the water contact angle (Figure 6a). The water contact angle of the PC 56 T 44 /SWCNT composite film before irradiation was 93 • . This result indicates that the surfaces of the SWCNTs were covered with PC 56 T 44 through noncovalent functionalization. In contrast, the water contact angle after irradiation at 313 nm was 43 • . Since the water contact angle for the pristine SWCNTs was around 85 • [24], the change in the surface composition of the SWCNT film led to a change in the water contact angle. We also found that the formation of COOH groups upon photocleavage increased surface hydrophilicity (Figure 6b). Therefore, the composite film became hydrophilic after photocleavage of the hydrophobic solubilizing unit.

Modulation of the Electrical Conductivity of the PC56T44/SWCNT Composite Film upon Photocleavage
Finally, the electrical conductivity (σ) of the photocleaved composite film was investigated. Since conductive SWCNTs form network structures in films, an applied voltage generates electrical current through the nanotube junctions [25]. A PC56T44/SWCNT composite film was fabricated on interdigitated gold electrodes by drop-casting a suspension in THF (Figure 7a). Before irradiation, the PC56T44/SWCNT composite film showed a σ value of 2.3 × 10 −3 S cm −1 (Figure 7(b1)). On the other hand, a 38-fold increase in the current was observed after photoirradiation at 313 nm (8.8 × 10 −2 S cm −1 ; Figure 7(b2)). The σ value for the PC56T44/SWCNT composite was higher than that for the poly(dioctylfluorene)/SWCNT composite (4.0 × 10 −7 S cm -1 ) [26] and the poly(3-octylthiophene)/SWCNT composite (4.7 × 10 −4 S cm -1 ) [27]. This comparison suggests that our composite is superior for the high σ material. In contrast, the PT/SWCNT composite film exhibited a σ value of 7.7 × 10 −9 S cm −1 ( Figure S3). This low σ value may be attributed to insufficient junctions between nanotubes, owing to the poor dispersion behavior of the PT/SWCNT composite (Figure 3b).
These results suggest that the increase in σ was due to the photocleavage of the solubilizing unit. Because the solubilizing unit is electrically insulating, photocleavage led to a higher σ value for the PC56T44/SWCNT composite film. Photoinduced modulation of the σ value is attractive for the development of SWCNT-based film devices such as photovoltaics and thermoelectric devices.

Modulation of the Electrical Conductivity of the PC 56 T 44 /SWCNT Composite Film upon Photocleavage
Finally, the electrical conductivity (σ) of the photocleaved composite film was investigated. Since conductive SWCNTs form network structures in films, an applied voltage generates electrical current through the nanotube junctions [25]. A PC 56 T 44 /SWCNT composite film was fabricated on interdigitated gold electrodes by drop-casting a suspension in THF (Figure 7a). Before irradiation, the PC 56 T 44 /SWCNT composite film showed a σ value of 2.3 × 10 −3 S cm −1 (Figure 7(b1)). On the other hand, a 38-fold increase in the current was observed after photoirradiation at 313 nm (8.8 × 10 −2 S cm −1 ; Figure 7(b2)). The σ value for the PC 56 T 44 /SWCNT composite was higher than that for the poly(dioctylfluorene)/SWCNT composite (4.0 × 10 −7 S cm −1 ) [26] and the poly(3-octylthiophene)/SWCNT composite (4.7 × 10 −4 S cm −1 ) [27]. This comparison suggests that our composite is superior for the high σ material. In contrast, the PT/SWCNT composite film exhibited a σ value of 7.7 × 10 −9 S cm −1 ( Figure S3). This low σ value may be attributed to insufficient junctions between nanotubes, owing to the poor dispersion behavior of the PT/SWCNT composite (Figure 3b).

Modulation of the Electrical Conductivity of the PC56T44/SWCNT Composite Film upon Photocleavage
Finally, the electrical conductivity (σ) of the photocleaved composite film was investigated. Since conductive SWCNTs form network structures in films, an applied voltage generates electrical current through the nanotube junctions [25]. A PC56T44/SWCNT composite film was fabricated on interdigitated gold electrodes by drop-casting a suspension in THF (Figure 7a). Before irradiation, the PC56T44/SWCNT composite film showed a σ value of 2.3 × 10 −3 S cm −1 (Figure 7(b1)). On the other hand, a 38-fold increase in the current was observed after photoirradiation at 313 nm (8.8 × 10 −2 S cm −1 ; Figure 7(b2)). The σ value for the PC56T44/SWCNT composite was higher than that for the poly(dioctylfluorene)/SWCNT composite (4.0 × 10 −7 S cm -1 ) [26] and the poly(3-octylthiophene)/SWCNT composite (4.7 × 10 −4 S cm -1 ) [27]. This comparison suggests that our composite is superior for the high σ material. In contrast, the PT/SWCNT composite film exhibited a σ value of 7.7 × 10 −9 S cm −1 ( Figure S3). This low σ value may be attributed to insufficient junctions between nanotubes, owing to the poor dispersion behavior of the PT/SWCNT composite (Figure 3b).
These results suggest that the increase in σ was due to the photocleavage of the solubilizing unit. Because the solubilizing unit is electrically insulating, photocleavage led to a higher σ value for the PC56T44/SWCNT composite film. Photoinduced modulation of the σ value is attractive for the development of SWCNT-based film devices such as photovoltaics and thermoelectric devices.  These results suggest that the increase in σ was due to the photocleavage of the solubilizing unit. Because the solubilizing unit is electrically insulating, photocleavage led to a higher σ value for the PC 56 T 44 /SWCNT composite film. Photoinduced modulation of the σ value is attractive for the development of SWCNT-based film devices such as photovoltaics and thermoelectric devices.

Conclusions
We have successfully developed a photocleavable polymer dispersant PC 56 T 44 for SWCNTs. Ultrasonication yielded dispersed PC 56 T 44 /SWCNT composite in THF through noncovalent functionalization. A solution-processed PC 56 T 44 /SWCNT composite film exhibited photocleavage of the solubilizing unit upon irradiation. Water contact angle measurements revealed that the surface properties of the composite film changed from hydrophobic to hydrophilic upon irradiation due to the formation of hydrophilic carboxyl groups. Furthermore, the photocleaved PC 56 T 44 /SWCNT composite film showed a dramatic increase in electrical conductivity because the solubilizing unit in PC 56 T 44 acts as an electrical insulator.
A particularly important aspect of this study is the increase in electrical conductivity when the solubilizing unit was photocleaved to yield an insoluble SWCNT film. Since the photoirradiated area is insoluble, selective irradiation can form a patterned SWCNT film by solution processing. We believe that such patterned films have potential applications in printed electronics, such as photovoltaics and thermoelectric devices with p-n heterojunctions.